Corrosion is defined as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. Corrosive attack begins on the surface of the metal. The corrosion process involves two chemical changes. The metal that is attacked or oxidized undergoes an anodic change, with the corrosive agent being reduced and undergoing a cathodic change. The tendency of most metals to corrode creates a major maintenance challenge for metals and metal products, particularly in areas where adverse environmental or weather conditions exist.
Corrosion of aluminum is a major problem for the aerospace and many other industries. One current technique to mitigate this problem involves anodization of the aluminum followed by an immersion seal. Typically, this seal involves the use of hexavalent chrome, which is increasingly regulated. Chromium-based anti-corrosive systems containing hexavalent chromium compounds have proven to be an extremely useful and versatile group of chemistries that are extensively used in aircraft metal treatment processes. They impart many beneficial anti-corrosive characteristics to metallic substrates on which they are applied and have been used extensively for the pre-treatment of metals before coating, adhesive bonding, and surface finishing.
Concern about chromium—and in particular, hexavalent chromium—in the environment has generated a need to replace chromium-based systems. Therefore environmentally preferable, commercially acceptable alternatives to chromium-based systems are a welcome addition to corrosion prevention coatings.